Climate warming accelerates cotton growth while cultivar shifts extend the growth period

Temperature changes and cultivar shifts (cultivar renewal or cultivar replacement) seriously affect cotton phenology and production under climate change but the specific effects remain uncertain. Here, we combined cotton phenological observation data and corresponding meteorological data for approximately twenty years from 56 sites to explore the effect of cultivar shifts on cotton phenology across China. Phenological growth models were used to investigate how these factors influenced cotton phenology in the Yellow River basin, Yangtze River basin and Northwest inland three major cotton regions in China. As a result, the duration of the whole cotton growing period (GPw) was prolonged at 87.5% of stations, although the mean temperature (Tmean) during GPw increased at 98.2% of stations. We demonstrated that once the cultivar effect was fixed, the increase in temperature alone produced a general advancement regarding the dates of emergence, squaring, flowering, boll opening and harvesting, leading to a shortening of the corresponding growth period of cotton by 1.74, 2.2, 1.74 and 2.31 days/decade, respectively, and a shortened duration of the whole growth period (GPw) by 4.06 days/decade. In contrast, cultivar shifts prolonged the duration of emergence-squaring, boll opening-harvest and GPw at 32 (57.1%), 46 (82.1%), and 33 (58.9%) stations by an average of 5.13, 11.61 and 6.29 days/decade, respectively, although the durations of the squaring-flowering and flowering-boll opening periods were reduced. However, the difference was that cultivar shifts reduced the length of the GPw of cotton in the Yellow River basin by 1.35 days/decade. This result indicated that, differences in the effect of cultivar shifts on cotton phenology in different cotton regions. Over the last decades, the introduction of new varieties requiring longer heat times in the Yangtze River basin and the Northwest inland cotton region compensated for some of the cotton phenological changes caused by increased temperatures, while early maturing and resistant cotton varieties were more suitable for selection in the Yellow River basin.